The present invention relates to printing plates, and more particularly, to such plates for use in the fabrication of electronic circuitry.
In conventional semiconductor device manufacturing processes, sheets of metals, semiconductors, dielectrics and other materials are deposited uniformly across a substrate. Each layer is then patterned using wet or dry etching, typically using photolithographically defined photoresist layers as masks. These processes are complex and have a relatively low throughput.
Printing techniques enable materials to be directly deposited onto the substrate in a desired pattern, affording a much higher throughput of substrates. One such technique employs gravure or intaglio printing plates which have the pattern to be printed sunk into their surface.
Existing intaglios for printing images or text onto paper are normally formed of either a metal block or a polymer plate mounted onto a metal sheet. However, the use of intaglios made from these materials for the manufacture of electronic circuitry may be problematic. A metal plate is relatively resistant to wear during use, but when printing fine features on a scale required for electronic circuitry, for example line widths below 20 xcexcm, the grain structure of the metal may have a detrimental effect on the printing quality. A polymer plate does not produce these grain structure artefacts, but its wear resistance is poor.
An additional consideration when printing fine features is the relative expansion properties of the printing plate and the substrate. If they are not sufficiently closely matched, temperature fluctuations may cause misalignment between different layers printed on the substrate. For example, it is desirable to be able to print circuits onto a glass substrate to fabricate active matrix liquid crystal displays (AMLCDs). However, the thermal expansion properties of a metal plate would be quite different to those of the glass substrate. A polymer plate would be a better match, but this difference may still have a substantial effect. A printing plate formed of the same glass as the substrate would be an ideal match. However, it is not practicable to define and etch features of sufficiently high resolution as dry etching of such glasses is too slow, and the isotropic nature of wet etching limits the minimum feature size.
The present invention provides a printing plate for use in fabricating electronic circuitry on a substrate, comprising a body layer, and a non-metallic pattern definition layer over the body layer having a printing pattern anisotropically etched into its outer surface. The material forming the body layer and thus the bulk of the plate may therefore be selected to provide a sufficiently accurate match to the thermal expansion properties of the substrate, whilst the material forming the pattern definition layer may be chosen for its etch properties. In this context, a sufficiently accurate match of thermal expansion properties or reference to two materials having substantially the same thermal expansion properties concerns their properties over the temperature range to which the printing plate and substrate may be subjected during the printing process, the match being sufficiently close for any relative expansion to have a negligible effect on the alignment of printed patterns over that temperature range.
The use of an anisotropic etch process enables the formation of a pattern having relatively steep side walls which in turn allows fine features to be printed. Preferably, the angle between a normal to the plate and the upper portion of the side walls of the pattern (referred to as the release surface angle) is approximately 25xc2x0 or less. An angle of around 10xc2x0 or less is preferable for particularly fine patterns.
The pattern definition layer may comprise material selected from polyimide, silicon dioxide, silicon nitride, and sol gel materials. The plate may also include a wear resistant layer over the pattern definition layer, which is more resistant to wear than the pattern definition layer.
In a preferred embodiment, the body layer is formed of material selected substantially to match the thermal expansion properties of the substrate. The body layer may comprise glass or quartz.
The present invention further provides a method of forming a printing plate for use in fabricating electronic circuitry on a substrate, comprising providing a body layer, depositing an additional non-metallic layer over the body layer, providing a mask layer over the pattern definition layer, and anisotropically etching a printing pattern into the outer surface of the pattern definition layer.
The step of providing a mask layer preferably comprises the steps of depositing a metal layer over the pattern definition layer, and patterning the metal layer.
The invention additionally provides a method for fabricating electronic circuitry on a substrate using a printing plate of the form described above, wherein the body layer of the plate is formed of material having thermal expansion characteristics substantially the same as those of the substrate. Preferably, the body layer is formed of the same material as the substrate.